well i have to say that i would not geld him, you limit who will buy him. if someone wants him as a gelding they can always do that, but once you geld him, then he is ALWAYS a gelding! So look at it like that.
Then you will have to add the gelding fee to the bottom dollar of what you have in him. I think you said it cost you about 15-20 a month to keep him. I have kept a few fillies over the years to replace some of my older mares, and it cost me much more than that per baby per month , but i feed all they can eat safe choice and Alfalfa hay, and worm them till they are a year old once a month, which makes my yearlings look like most peoples two year olds! but i realize after buying several horses this year that people dont feed and worm like i do! I like my horses well fed and cared for, because they produce better, and feel better when in optimal health. So what i am getting at is, the longer you hold on to him, the more you have in him, dont limit your buyers, and above all, dont get greedy!!! you will live to regret that! Been there done that! take the highest offer, if you are wanting to sell him. If you dont get what you want, look at it like "It wont hurt long, and a couple of glasses of nice semi sweet blush wine always seems to help take the sting out of selling horses!